Radiation absorbing surfaces

ABSTRACT

A substantially radiation absorbing layer of metal having a microstructured surface characterized by a plurality of randomly positioned discrete protuberances of varying heights and shapes, which protuberances have a height of not less than 20 nanometers nor more than 1500 nm, and the bases of which contact the bases of substantially all adjacent protuberances is disclosed. The metal layer, which may be a coating on a variety of substrates, is useful as a radiation absorber (particularly solar). A method is disclosed for producing such layers.

This application is a divisional application of copending U.S. Ser. No.278,979, filed June 29, 1981, now U.S. Pat No. 4,396,643.

TECHNICAL FIELD

The present invention relates to articles having novel radiationabsorbing surfaces and to a method for producing such surfaces on avariety of substrates. These radiation absorbing surfaces are useful insolar absorbers and in fabricating graphic arts films.

BACKGROUND ART

There is currently considerable interest and research into ways toutilize solar energy to help overcome society's present dependence on adiminishing supply of fossil fuels and problem-ridden nuclear fuels.Alternative non-polluting energy sources such as solar energy would behighly desirable. To make solar energy economically feasible, highlyefficient radiation absorbing surfaces are needed.

A number of radiation absorbing surfaces are known in the prior art.They may have selective or non-selective characteristics. A selectiveradiation absorbing surface is one which has high absorptivity and lowreflectivity in the solar radiation range and low emissivity and highreflectivity in the thermal infrared region of the electromagneticspectrum. For a non-selective surface, the absorption and emissionproperties do not change appreciably in the above two spectral regions.

At least three well-known methods of preparing selective black radiationabsorbing surfaces exist in the art. One method involves the coating ofa metal base with a thin film that absorbs solar radiation but istransparent to infrared radiation. The high infrared reflectivity of thebase provides the low infrared emissivity. A second method requires thecoating of an opaque metal or metallic oxide having a high infraredreflectivity but a low solar reflectivity with antireflection layerssuch that the solar reflectivity is further reduced, thereby enhancingthe solar absorption. Finally, a third method of obtaining a selectiveblack absorber is to fabricate a wire-mesh with appropriate dimensionssuch that solar radiation, but not infrared, is trapped. An example of aselective radiation absorbing surface is disclosed in U.S. Pat. No.4,148,294 which teaches a solar energy absorbing panel having metalbodies projecting longitudinally out from spaced pores in a substrate.

Non-selective surfaces may be produced by utilizing coatings of paint orenamels. Black paints are preferred and are generally composed of carbonin a binder, or a combination of carbon and calcium phosphate in abinder material. Other black coatings useful as non-selective absorbersare disclosed in U.S. Pat. No. 2,891,879 wherein finely divided aluminumis mixed with an organopolysiloxane and, if desired, further mixed witha suitable solvent such as butyl acetate or toluene. These materials arecoated on suitable substrates by a variety of methods, such as spraying,dipping, painting, knife coating, spinning, and printing. Anon-selective black, non-shiny and substantially non-reflective surfaceis disclosed in U.S. Pat. No. 4,138,262 wherein bismuth is firstsputtered and then vacuum evaporated onto a flexible plastic substratefollowed by a coating of photoactive material. The coated material isthen utilized as an imaging film.

Selective radiation absorbing surfaces known in the art suffer from avariety of shortcomings. These are problems of metal grain particleagglomeration, temperature instability, undesired chemical reaction, andhigh cost of preparation of the coatings. Non-selective absorbingsurfaces often suffer from high temperature instability, poorweatherability, and inefficient absorption over an extended wavelengthrange.

Assignee's U.S. Pat. No. 4,426,437, issued Jun. 17, 1984, disclosesimageable articles utilizing the radiation absorbing composite surfacesof the present invention.

DISCLOSURE OF INVENTION

The present invention overcomes many of the prior art problems byproviding an article exhibiting appreciable radiation absorbance(particularly solar), high temperature stability, good weatherability,and which is economically produced. The novel radiation absorbingsurfaces have substantially selective characteristics. In anotheraspect, a method is provided for preparing such surfaces on a variety ofsubstrates.

The present invention provides a substantially radiation absorbing layerof metal having a microstructured surface characterized by a pluralityof randomly positioned discrete protuberances of varying heights andshapes, which protuberances have a height of not less than 20 nanometersnor more than the wavelength of the radiation absorbed, i.e., 1,500nanometers, and the bases of which contact the bases of substantiallyall adjacent protuberances. The tips or apices of the protuberances arespaced apart a distance in the range of 3 to 500 nm. The average spacingbetween the tips can be no more than twice the average height of theprotuberances, and preferably it is no more than half the averageheight. The most preferred average spacing between the tips is in therange of 1/10 to 1/4 the average height of the protuberances.

In the preferred embodiment, the articles of the present inventioncomprise a substrate, a microstructured metal oxide (or hydroxyoxide)layer, and an overcoating of a thin film of at least one metal. In asecond embodiment, the article has a microstructured replicatedcomposite surface of a material such as plastic, overcoated by athin-film of at least one metal. In a third embodiment, the article is areplicated microstructured surface of metal. In the preferredembodiment, the articles of the present invention comprise a substratewhich may be of virtually any construction, i.e., transparent or opaque,insulative, semiconductive or metallic, having a flat, curved or complexshape, and having formed thereon a non-absorbing metal oxide coating,the metal being selected from the group consisting of aluminum,magnesium, zinc, or alloys thereof. The oxide layer is formed by thesubstantially complete conversion of a thin-film of a metal or metalalloy, the thickness of which thin-film prior to conversion was in therange of 5 to 200 nm. The thickness of the thin-films can vary withinthese same limits over the surface of the article. Thus, careful controlof the thickness of the starting thin-film is not necessary. The oxidelayer exhibits a surface characterized by a plurality of randomlypositioned discrete protuberances of varying heights and shapes, whichprotuberances extend from the surface of said substrate a distance ofnot less than 20 nm, nor more than the wavelength of the radiationabsorbed, i.e., 1,500 nanometers, and the bases of which are insubstantial contact with the bases of each of adjacent protuberances.The metal oxide layer is overcoated by a thin-film of metal from 40nanometers to 200 nanometers thick, such that the outer surface of themetal coating substantially conforms to the shape of the structuredsurface of the oxide layer without completely filling in the valleys andvoid spaces. When so structured, and without the use of dyes orpigments, the article exhibits absorbance over the total solar range of70 percent to 98 percent.

A method for generating the absorbing surfaces of the preferredembodiment comprises the steps of forming a microstructured layer on asubstrate, which method is disclosed in U.S. Pat. No. 4,252,843 which ishereby incorporated herein by reference, and then depositing a thinlayer of metal on the microstructure in such a manner that the depositedmaterial conforms to the structure and closely replicates theunderlaying topography in the exterior surface of the deposited materialfilm.

Alternatively, the microstructure of the metal oxide layer may bereplicated into the surface of a second element and the replicatedmicrostructured surface overcoated with a thin film of at least onemetal from 40 nm to 200 nm thick to produce a radiation absorbingarticle. If the replicated surface is itself metallic with amicrostructure according to the teachings of this invention then it neednot be overcoated, although it may be, with a thin-film of metal.

"Microstructure" refers to the rough and structured topography resultingfrom the conversion of a metal thin-film to an oxide or hydroxide layeror replica thereof by a chemical or chemical/electrochemical method. Thereplication can take place in materials such as plastics which are thenovercoated with metals, or the replication may be directly into a metal.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIG. 1 is an enlarged sectional view of the preferred embodiment of thepresent invention showing a microstructured oxide layer on a substrateovercoated by a thin-film of metal;

FIG. 2 is an enlarged sectional view of a second embodiment of thepresent invention depicting a substantially radiation absorbingcomposite structure comprising a replicated microstructured surfaceovercoated by a thin-film of metal;

FIG. 3 is an enlarged sectional view of a third embodiment of thepresent invention showing a replicated microstructured metallic surface;

FIG. 4 is a graph of the reflectance, before (curve A) and after (curveB) heat treatment, as a function of electromagnetic radiation wavelengthfor a radiation absorbing surface using a chromium overcoating 75.9 nmthick according to the present invention;

FIG. 5 is a graph of the reflectance, before (curve A) and after (curveB) heat treatment, as a function of electromagnetic radiation wavelengthfor a radiation absorbing surface using a different metal thickness(i.e., 100 nm) according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides, in a preferred embodiment, an articlehaving a substantially radiation absorbing surface, said articlecomprising:

a substrate having on at least one surface thereon, a layer of an oxideof a metal selected from the group consisting of aluminum, magnesium,zinc, or alloys thereof, which layer is a substantially completeconversion of a thin-film of the metal, the thickness of the thin-filmprior to conversion being at least 5 nanometers and exhibiting a surfaceafter the conversion characterized by a plurality of randomly positioneddiscrete protuberances of varying heights and shapes, whichprotuberances extend from the substrate surface a distance of not lessthan 20 nanometers nor more than the wavelength of the radiationabsorbed, i.e., 1,500 nanometers, and the bases of which contact thebases of substantially all adjacent protuberances, and

an overcoating upon the oxide layer, which overcoating is a contiguousconnected thin layer of metal having a thickness in the range of 40 to200 nanometers.

The articles having novel radiation absorbing surfaces and the methodfor producing such surfaces on a variety of substrates such as aluminum,stainless steel, glass, polycarbonate, polyacetate, or polyester, canbest be understood by referring more particularly to the drawing. Aprior art article comprising a substrate upon which is formed amicrostructured oxide layer is disclosed in U.S. Pat. No. 4,190,321,incorporated herein by reference. Referring now to FIG. 1, there isshown the novel radiation absorbing article 20 of the present invention,the article comprising substrate 22 upon which is formed microstructuredoxide layer 24 which is overcoated with a thin layer of metal 26. In theprocess for producing the preferred embodiment of the present invention,article 20 is formed by depositing a thin metal film on the prior artembodiment such that the metal coating conforms to the shape of thestructured surface of the substrate without filling in the valleys andvoid spaces. Article 20 is an excellent radiation absorber. Before it isovercoated, microstructured oxide layer 24 exhibits a surface morphologywhich can generally be described as being a plurality of randomlypositioned discrete protuberances of varying heights and shapes, thebase of each protuberance being in substantial contact with the base ofadjacent protuberances. The protuberances extend from the substratesurface a distance of not less than 20 nanometers, and preferably extendfrom the substrate surface a distance varying from that corresponding tothe wavelength of the radiation absorbed down to 1/10 that wavelength,i.e., approximately 1500 down to 40 nanometers.

FIG. 2 shows a second embodiment 30 of the present invention wherein themicrostructure of the above-mentioned metal oxide layer is replicatedinto the surface of a second element 32, which may be an embossable orcastable material, metal, nonmetal, or organic. The replicatedmicrostructured surface 32 is then overcoated by a thin-film of metal 26to produce an article of the present invention having a substantiallyradiation absorbing surface.

Thin film 26 covering microstructures 24 and 32 can be selected from avariety of suitably stable metals; preferably it is chromium, aluminum,copper, gold, or nickel; and most preferably it is chromium. Metalalloys may also be used. Thin film 26 can be deposited by vacuum vapordeposition, sputtering, electroless plating, chemical vapor deposition,or other suitable methods. A single film or a set of films usingdifferent materials may be used. All metals give selective surfaces,although some are better than others depending on individual metalemissivity. The choice of the material for the substrate helps determinethe amount of absorbed energy which will be reradiated. Suitablesubstrate materials to produce low radiation emitters are metals such asstainless steel, aluminum, chromium, copper, and nickel. Materials whichare high thermal radiators are dielectrics such as glass and ceramics.Thin layer 26 has a thickness in the range of 40 to 200 nanometers,preferably 40 to 160 nm and most preferably 50 to 80 nm.

FIG. 3 shows a third embodiment 40 of the present invention wherein thereplicated microstructured surface 42 is itself metallic (single ormultiple layered) and is a self-supporting article according to thepresent invention which is substantially radiation absorbing.

Articles having the novel radiation absorbing surfaces of the presentinvention include selective solar absorbers, for example, flat platesolar collectors, and, in combination with photoactive materials, areuseful as silverless graphics art films.

Objects and advantages of this invention are further illustrated by thefollowing examples, but the particular materials and amounts thereofrecited in these examples, as well as the conditions and details, shouldnot be construed to unduly limit this invention.

EXAMPLE 1

A glass slide was cleaned and then a portion was vapor coated with 30 nmof aluminum on one side. The aluminum film was then subjected tosaturated steam at a temperature of about 95° C. for approximately threeminutes, during which time the film was completely converted to amicrostructured layer of boehmite, AlO(OH), approximately 120 nm thick.The slide was subsequently rinsed, dried, and then the coated side wasovercoated with a coating about 160 nm thick of vapor depositedchromium. The area on the glass slide where the chromium was depositedover the structured layer of boehmite was black and non-reflective andwas a very good absorber. The area where the chromium was depositeddirectly on glass was shiny and highly reflective and was a very poorabsorber.

EXAMPLE 2

A 200 micron thick clean foil of #3003 aluminum alloy (available fromAlcoa) was exposed to saturated steam as in Example 1 for about 2 or 3minutes to generate a layer of boehmite and then was overcoated on onemajor surface by vacuum vapor deposition of a coating of approximately51 nm thick of chromium. The surface appeared black and was shown to benon-reflecting and a good absorber. The emissivity, E, was measured andfound to be 0.19 which demonstrated that it was a poor emitter and thusdid not reradiate the absorbed energy.

EXAMPLE 3

A radiation absorbing surface was prepared as in Example 2 with theexception that the chromium layer was about 87.5 nm thick. The surfaceagain appeared black and was shown to be non-reflecting. The measuredemissivity was 0.38 which demonstrated again that most of the absorbedenergy was not reradiated.

EXAMPLES 4 and 5

Radiation absorbing surfaces were prepared as in Example 2 with theexception that the chromium layers were about 75.9 nm and 100 nm thick,respectively. The surfaces again appeared black and were non-reflectingand the emissivities measured 0.28 and 0.38, respectively. The sampleswere then heated in air for 68 hours at a temperature of 300° C. Thereflectivities as a function of wavelength were measured before (curveA) and after heat treatment (curve B) and are shown in FIGS. 4 (75.9 nmthick chromium overcoating) and 5 (100 nm thick chromium overcoating).The emissivity after heat treatment was 0.17 and 0.21, respectively.

The increase in reflectivity and the shifts in the spectral positions ofthe reflection maxima and minima may be related to oxidation (decreasein thickness) of the chromium layer. The decrease of emissivityassociated with increased reflectivity is in accord with Kirchhoff's lawwhich states that E=1-R, where R is the reflectivity of the surface.This emissivity versus metal film thickness relationship was alsoobserved in the absence of heat treatment, as Examples 2-5 show. Thatis, E was lower for thinner chromium films.

EXAMPLE 6

A portion of one surface of a polyester film was vacuum vapor coatedwith a layer of aluminum approximately 50 nm thick and subsequentlyconverted to boehmite by exposure to steam as in Example 1. Electronmicrographics of the converted layer on polyester show it to bestructured and composed of pointed protuberances extending upward fromthe surface to a height of approximately 120 nm with the tips spacedabout 20 nm apart. A 125 micron thick film of low density polyethylenewas then placed on the microstructured surface and subsequentlylaminated between two plattens heated to 125° C. and pressed together ata pressure of about 141 kg/cm² for two minutes. After cooling, theembossed polyethylene film was peeled from the boehmite surface and thenovercoated on the embossed side with 52 nm of chromium. The surfaceappeared very black and was a good absorber where the boehmitemicrostructure portion had been embossed into the polyethylene andbright, shiny metallic and a poor radiation absorber where thepolyethylene was smooth.

EXAMPLE 7

A layer of boehmite was formed on a thin aluminum foil by immersing itfor ten minutes in water at 70° C. and pH 8.65. After rinsing the foilin distilled water, a layer of nickel was deposited on one side on theboehmite structure using an electroless method described in U.S. Pat.Nos. 3,666,527 or 3,690,944. The surface of the foil on the side onwhich the nickel was deposited appeared black. The measured reflectivityof the surface was less than 4% in the wavelength range between 380 nmand 700 nm; the emissivity was 0.52. The measured values indicated thatan absorber with substantially selective surface properties had beenproduced.

EXAMPLE 8

A 60 nm thick film of magnesium was vapor coated onto one side of apolyester sheet. The sample was then immersed in deionized water at 55°C. to convert the magnesium metal film to a microstructured layer ofhydrated oxide. The microstructured layer was then overcoated by vacuumvapor deposition with a 45 nm thick film of copper. The surface appearedblack where the copper was deposited over the microstructure and it wasa good absorber.

EXAMPLE 9

A 225 micron thick foil of #5352 aluminum alloy (available from Alcoa)was etched in aqueous 2.5% NaOH solution at 54° C. for 30 seconds,rinsed and exposed to steam for three minutes. Then 80 nm of chromiumwas coated on the structured surface by vacuum vapor deposition. Thepanel appeared dark gray. The panel was then dipped for 20 seconds inaqueous KMnO₄ solution at 73° C. to oxidize the chromium surface. Thepanel appeared dark blue-black and its absorption was enhanced.

EXAMPLE 10

A 30 cm×30 cm×1.25 cm piece of acrylic sheet was cleaned with a milddetergent, rinsed with distilled water and dried. A 45 nm thick layer ofaluminum was deposited on the clean surface via vacuum metallization.The aluminized surface was then exposed to steam for about five minutesto convert it to microstructured aluminum oxide (boehmite). Themicrostructure was then replicated in metal by vacuum metallizing thesurface first with chromium (20 nm) and then gold (300 nm). In additionto replication, this step produced a surface which was also conductiveand served as the cathode for the next step which was to produce a heavymetal electroform backing. The acrylic sheet substrate was then removedby shearing at the acrylic sheet substrate/boehmite microstructureinterface. The boehmite microstructure was next removed from thereplicated metal surface via an acid etch treatment followed by adistilled water rinse and dried. This treatment left a replica of theboehmite microstructure in the chromium layer.

The absorber thus obtained had a measured emissivity of 0.123 and anabsorption in the visible region of the solar spectrum of greater than98% at 400 nm, decreasing to greater than or equal to 91% at 700 nm.This was comparable to selectivity claimed by other commercial processessuch as the black nickel and black chrome electroplates.

Various modifications and alterations of this invention will becomeapparent to those skilled in the art without departing from the scopeand spirit of this invention, and it should be understood that thisinvention is not to be unduly limited to the illustrative embodimentsset forth herein.

What is claimed is:
 1. A method for absorbing radiant energy by a metalhaving a radiation absorbing surface said method comprising thesteps:(a) preparing a metal having a substantially radiation absorbingsurface by a process comprising the steps:(1) providing a substratehaving on at least one surface thereof a first microstructured layer,said layer being an oxide or hydroxyoxide of a metal, selected from thegroup consisting of aluminum, magnesium and zinc or alloys thereof,which layer is a substantially complete conversion of a thin-film ofsaid metal, the thickness of said thin-film prior to conversion being atleast 5 nanometers and exhibiting a surface after said conversioncharacterized by a plurality of randomly positioned discreteprotuberances of varying heights and shapes, which protuberances extendfrom said substrate surface a distance of not less than 20 nanometersnor more than 1500 nm, and the bases of which contact the bases ofsubstantially all adjacent protuberances, (2) replicating said firstmicrostructured layer by embossing, casting, or vacuum metallizing ontothe surface of a second layer, said second layer being a metal, toprovide said metal with a microstructured surface of randomly positioneddiscrete protuberances, and (3) removing said second layer from saidfirst microstructured layer to provide said metal with a substantiallyradiation absorbing surface of randomly positioned discreteprotuberances, and (b) orienting said radiation absorbing surface suchthat said microstructured surface is contacted with radiant energy so asto absorb radiant energy.
 2. The method according to claim 1 whereinsaid metallized second layer is selected from chromium, aluminum,copper, gold, nickel, and metal alloys.
 3. The method according to claim1 wherein the tips of said protuberances are spaced apart a distance inthe range of 3 to 500 nm.
 4. The method according to claim 1 wherein theaverage spacing between the tips of said protuberances is in the rangeof 1/10 and 1/4 the average height of the protuberances.
 5. The methodaccording to claim 1 wherein the radiant energy absorbance by said metalover the total solar range of said article is within the range of 70percent to 98 percent.
 6. The method according to claim 1 wherein saidmetal is a selective solar absorber.
 7. The method according to claim 1wherein said metal is a flat plate solar collector.